-rw------- 1 tc staff 207921152 Aug 8 08:09 blkindex.dat
-rw------- 1 root staff 425711616 Aug 8 06:53 blk0001.dat
The permissions are on the left. They are
(1): (-) normal file
(234): (rw-) Owner can read and write (but not execute as a program)
(567): (---) Other members of group cannot read or write
(890): (---) Non-group members cannot read or write
It seems that the owner was changed on the file, but not the group. You can do one of two things, either give permission for all group members to be able to read the file, and/or change the file owner back to tc.
To change the owner, you would use the chown command like so:
sudo chown tc blk0001.dat